Advances in combustion turbines over the past several years have made natural gas fired, gas turbine, combined cycle plants efficient, clean and reliable. Because of this, they have become the preferred new power generating alternative for locations where natural gas is readily available, e.g., via a pipeline. The cost of power from the new natural gas combined cycle plants is currently approximately 60% of that obtained from pulverized coal-based plants of equal or larger scale even where coal is delivered at 60% of the price of natural gas.
Bailie, U.S. Pat. No. 3,853,498, describes an indirectly heated biomass gasifier which uses hot bed material from a stationary (bubbling fluid bed) exothermic char combustor to indirectly dry and devolatilize biomass fed to an endothermic stationary (bubbling bed) devolatilizer (also known as gasifier or pyrolizer). Because of the high volatile content of biomass used in the Bailie process (above 80% by weight), most or all of the char combustion exothermic heat is required to supply the endothermic requirements of the devolatilizer. For this reason Bailie's biomass gasifier is not suitable for coal. It is not integrated into a combined cycle power plant.
Feldmann et al., U.S. Pat. No. 4,828,581, describe an improvement over the Bailie patent for a specific indirectly heated biomass gasifier/devolatilizer vessel design. The '581 patent describes a two zone indirectly heated biomass devolatilizer. The initial fluidization velocity, like that of Bailie, is in the stationary or bubbling region (below 7 feet per second (FPS)) using steam or recycled product gas for initial fluidization. Once high volatile biomass feed is introduced, however, to the top of this first zone, the rapid release of the large quantities of volatiles and moisture intrinsic to biomass produces velocities (above 15 FPS) sufficient to fully entrain the flow of bed material with residual char. The vessel used in the '581 patent has a higher length to diameter ratio than Bailie's devolatilizer (above 6:1). It also has a throughput per unit of vessel section area about 10 times that noted for stationary (bubbling) bed biomass gasifiers. Use of the '581 patent devolatilizer with coal or integration into a combined cycle power plant is not described. The two zone devolatilizer vessel design of Feldmann will not work with most coals. This is because coal provides insufficient moisture and volatile release to raise stationary bed (bubbling) fluidization velocities from below the 7 FPS claimed to above 15 FPS necessary for stable entrained flow. Even for those low rank, high moisture content coals where marginally sufficient volatiles may be released for entrained flow, a two zone gasifier vessel with "first space" velocities below the 7 FPS claimed adds complexity without compensatory advantages. Like Bailie, the vessel is not integrated into a combined cycle power plant.
Schemenau, U.S. Pat. No. 4,901,521, describes a coal fired combined gas turbine and steam turbine power plant utilizing either a circulating fluidized bed (CFB) boiler or a bubbling fluid bed boiler. In one embodiment, hot CFB boiler bed material is directly contacted with only a portion of the raw coal feed in the CFB bed return conduit. This technique does not create a fluidizing zone. As a result, only a portion of the coal volatiles available for use as gas turbine fuel are recovered. In another embodiment, a stationary (bubbling bed) coal combustor (not a CFB) is utilized together with a stationary (bubbling) fluid bed devolatilizer (which they call a degasifier/gasifier) in a manner similar to Bailie, but for coal, not biomass. This devolatilizer is fluidized with boiler exhaust gas containing CO.sub.2 and N.sub.2, thus substantially diluting the product gas Btu content.
Furthermore, use of a stationary fluid bed devolatilizer in Schemenau results in an order of magnitude lower throughput. Also, a product gas with higher tar and condensible liquids content due to-less thermal cracking than is possible with CFB devolatilizer results. Schemenau does not teach or suggest maximizing the ratio of higher efficiency gas turbine output to lower efficiency steam output because, (1) only a portion, not all of the raw coal is fed directly into the devolatilizer; (2) a portion of turbine exhaust gas sensible heat is not used to preheat turbine air compressor discharge prior to the turbine combustor (recuperator); (3) a portion of product gas sensible heat is not used to preheat gas turbine compressor discharge; (4) a portion of the fluid bed boiler bed material or exhaust gas sensible heat is not used to preheat gas compressor air discharge; (5) the devolatilizer lacks positive gas sealing means to prevent a portion of the volatile produced from escaping to the combustor or its exhaust; and (6) combustible gas production is not increased when necessary via the addition of steam and air or oxygen to the devolatilizer.
Gounder, U.S. Pat. No. 5,255,507, describes integrating a coal CFB boiler with a gas turbine cycle and using a recuperator in combination with an external fluid bed gas turbine air heater. The '507 patent does not, however, describe integrating a coal CFB boiler with a devolatilizer heated indirectly with the CFB boiler's hot bed material. The '507 patent does not teach or suggest maximizing the ratio of gas turbine cycle output to steam turbine cycle output because, (1) a portion of the raw coal is fed directly to the CFB boiler; (2) sensible heat from the gasifier is not used to preheat gas turbine Compressor air; and (3) sensible heat from the CFB boiler exhaust gas is not used to preheat gas turbine compressor air. In the '507 patent, a portion of the gas turbine fuel input is supplied by "a first fuel source" of natural gas, not coal gas. When natural gas is available via pipeline, the use of any coal based power technology today is seldom economic (due to 2 to 3 fold higher capital costs, 20% to 35% lower fuel efficiency and 2 fold higher operating and maintenance costs, all per Kwh of electrical output).
European Patent 607,795 to Dietz describes a CFB unit incorporated into a combined cycle system in a manner similar to Grounder.